This invention pertains to the technical field of physical sciences and technology. A novel detector is disclosed which can be used with instruments generating ionising radiations in a gaseous environment. This detector can be used also for particle measurements such as in the field of nuclear methods and instruments.
Conventionally, instruments employing electron and ion beams utilise a vacuum envelope into which the electron or ion beam is generated and applied. Such beams usually impinge on a specimen with which they interact and generate various products, or signals, such as electrons, ions, molecular fragments and x-rays or other photons. These instruments are used for imaging, testing, analysis, or modification of the surface of a specimen. To study a specimen, we need to detect the products of the beam-specimen interaction by suitable detection means usually placed inside the vacuum envelope of the instrument. However, a more recent generation of related instruments allows the examination of specimens inside a gaseous envelope with substantial pressure.
One electron beam instrument that allows the examination of a specimen in a gaseous environment is the environmental scanning electron microscope (ESEM) as disclosed by references 1 through to 10 below. This instrument employs a scanning electron beam that is generated in the vacuum envelope of an electron optics system. The vacuum envelope communicates with a specimen chamber via two small apertures which limit the flow of gas in the vacuum column to a negligible amount. Practically, most of the gas leaking through the first aperture from the specimen chamber is pumped out and only a very small amount of gas escapes into the vacuum of the electron optics column which is also pumped out and maintained in vacuum. The electron beam reaches the specimen through the two apertures with sufficient current still in a focussed condition. The focussed electron beam scans a very small surface area of the specimen in a raster form and releases various signals. One type of signal is electrons of low energy, called secondary electrons (SE). The SE are released in the gaseous environment and are detected with suitable means as disclosed by Danilatos in the references cited. Another type of signal generated is electrons of high energy, called backscattered electrons (BSE). The BSE, in turn, release free electrons of low energy from the gas molecules. The latter free electrons are also detected by suitable means as disclosed by Danilatos and summarised below.
The detection of free electrons of low energy in the gas of an ESEM is achieved by subjecting the electrons to a static electric field of sufficient intensity. As a result of the action of the external field, a controlled electrical discharge develops that multiplies the initial free electrons. The avalanche multiplication of the electrons is also accompanied by an avalanche of photon generation and multiplication in the gas. The discharge is controlled by the applied bias and develops as long as there is an initial supply of free electrons, whilst the discharge is extinguished when the supply source is eliminated. In other words, the amplified output signal is practically proportional to the input intensity of the source. The output signal is collected by suitable means, such as biased electrodes and photon detectors. In prior art, the same principle has been applied for detection and counting of ionising particles in particle physics, namely, by the proportional counter.
Although the principle on which proportional counters operate has been known art in the field of particle physics and nuclear instruments for a long time, the application of the same principle for the development of a detector in an ESEM has led to a series of new inventions and patents given by references 1 through to 6.
The present invention discloses a novel detector for free electrons of low energy in a gaseous environment of instruments employing electron and ion beams, or in the gaseous environment of a particle counter, in general. The novelty of this invention is primarily based on the introduction of an alternating electromagnetic field which can generate an electron and photon discharge in a gaseous environment. The use of an alternating field instead of a static field provides for novel and alternative means of detection for several instruments and has certain advantages over previous art.
The principle and application of an alternating electromagnetic field to create an electrodeless self-sustained discharge for completely different purposes has been previously presented by other workers as in references 11 through to 15. The same principle is also used for the generation of ion sources in various instruments. In all prior art, this principle has been used mainly in a self-sustained discharge mode for generating ions; however, it has never previously been disclosed or used as a proportional amplifier and detector of ionising radiations in a controlled discharge regime which takes place below the breakdown voltage of a self-sustained discharge.
This invention relates to a novel detection system for instruments employing focussed charged particle beams such as electron or ion microscopes and for technologies generally employing focussed electron or ion beams. Electron and ion microscopes are a sub-class of the latter instruments and ESEM is one form of an electron microscope. The ionising radiations emanating from the specimens under examination in those instruments as well as the ionising particles used in other fields of science can be detected by a novel method and apparatus disclosed by the present invention herein.
According to the present invention, a volume of gas is acted upon by an alternating (oscillating) electromagnetic field in the radiofrequency range, preferably but not limited, anywhere between 1 MHz and 1 GHz. Both the frequency and amplitude of oscillation are variable within a particular working range for each technological case. The working range depends on the nature of gas, pressure of gas and geometry of the detection volume. When one or more electrons are released in the gas by any source, the electrons are acted upon by the alternating field and are forced to oscillate and collide with the gas molecules or atoms. Some electron collisions ionise the gas molecules or atoms and create electron/ion pairs, while some other electron collisions excite the gas molecules or atoms and release photons. The electrons may also collide repeatedly with the surrounding walls generating new electrons. By such mechanisms the initial electron signal is multiplied in an avalanche form. The predominant mechanism of electron multiplication is determined by the chosen frequency, pressure, geometry and nature of gas, and the effects can be controlled accordingly. Now, it is an essential feature and requirement of the present invention that the amplitude of oscillation of the applied field is kept below the sparking value, i.e. below the point where the avalanche becomes self-sustained and uncontrollable. When this requirement is met, the discharge is controlled and is said to operate in the proportional region, where the amplified current is proportional to the initial number of free electrons released in the gas. The sparking value of the field depends on the chosen frequency, geometrical configuration, gas pressure and nature of gas. The preferred values of all these parameters depend on the instrument and application with which this invention is used and do not restrict the scope and spirit of the invention.
For the purposes of the present description of the invention the term xe2x80x9celectromagneticxe2x80x9d refers to either (a) the field generated between two electrodes biased with an alternating voltage and creating a predominantly alternating electric component of field with negligible magnetic component, or (b) the field created by an alternating electric current through a coil with both an alternating electric and magnetic component present.
In one form of the present invention, the amplified electric current (i.e. the electron avalanche) generated by the applied alternating field is collected by a set of electrodes properly biased to attract the electrons and ions, respectively. The collected current is further processed by known art in electronics circuitry.
In another form of the present invention, the amplified photons (i.e. the photon avalanche) concomitant with the electron avalanche are collected by a set of light guides and pipes leading to a photosensitive device, such as a photomultiplier. This variant form of signal collection is equivalent to the proportional scintillation counters used in particle physics, except that, herewith, the external field is oscillating instead of being static as in previous art. This constitutes a novel device which has several advantages over all prior art used in microscopy and related fields, and in particle physics, in general.
It is an object of the present invention to provide a novel detector which can be used in instruments employing a focussed charged particle beam. Usually, the beam (probe) is scanned over a specimen surface in a raster form and it spends a characteristic xe2x80x9cdwell timexe2x80x9d on every pixel element. The beam releases slow electrons from every pixel element of the specimen. The number of released electrons generally differs from pixel to pixel and characterises the variable properties of the specimen surface from point to point. According to the present invention, the specimen and its surrounding volume of gas is acted upon by an alternating electromagnetic field in the radiofrequency range. Both the frequency and amplitude of oscillation are variable in order to establish a working range for each case. The oscillating field of the present detector performs several oscillations during the dwell time over one pixel element. By the mechanism of the present invention described above, the initial number of free electrons multiply in an avalanche form. Similarly, an avalanche of photons is generated. The avalanche process is repeated for every pixel element of the specimen. Either the electron signal or the photon signal is collected and processed in the usual way by known art to form an image, or for other analysis and testing.
It is a further object of this invention to provide a detector for fast electrons released in instruments employing charged particle beams. The fast electrons released by the beam-specimen interaction (i.e. the BSE) traverse a volume of gas and collide with the gas molecules or atoms releasing, in turn, slow electrons. These slow electrons are subsequently multiplied by the mechanism of the oscillating field as disclosed in the present invention.
It is yet another object of this invention to provide a detector for any fast particles, in general, which can ionise the gas and release electro/ion pairs which are then subjected to the action of the alternating field as disclosed by the present invention.
The fundamental features of the present invention which constitute an inventive step are: (a) The use of an alternating electromagnetic field to act on electrons released in a gaseous volume by an incident radiation, (b) an avalanche multiplication of the initial electrons and an avalanche generation of photons, (c) the collection of electrons, or photons, or both, (d) the use of these mechanisms as a detection method and apparatus. The present system is governed by its own physical parameters, and its operation and use as a detector in scanned beam instruments has not been disclosed in any previous art. Likewise, its operation and use as a particle detector, in general, has not been used in any previous art.
It should be appreciated that there are various embodiments of this invention, all of which comprise the same set of fundamental features, but do not depart from the scope of the invention. It will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. All of these possible embodiments may vary among themselves to the degree that the present invention is adapted to various applications. Some of these variations are described first in general terms below.
The amplified electron/ion signal can be collected by suitable means. Such means can be the same electrodes with which we apply the oscillating electromagnetic field, in which case known modulation/demodulation techniques are used. Alternatively, the amplified electron/ion signal can be collected by separate electrodes.
All collection electrodes can be biased with an adjustable static (d-c) voltage in order to control the direction and amount of electron/ion flow. The alternating field can thus be superimposed with a d-c field in order to better define the detection volume and extract various signals.
Optionally and where possible, a static, or an alternating magnetic field can be added. The presence of a magnetic field causes the trajectories of the electrons to traverse a longer distance and thus increase the chances of collisions with gas molecules or atoms in cases of low gas pressure. For purposes of image quality, efficiency, gain, signal separation, charge and noise suppression, an array or system of electrodes of varying geometry, position and configuration is employed. The electrodes can be made in the form of plates, wires or meshes. The materials used for electrodes have suitable electrical properties and can be conductive, semiconductive and resistive materials.
In electron microscopes, it is necessary to protect the incident electron beam from possible influence of the alternating field, and suitable precautions are taken according to known art. For example, an ancillary electrode can be inserted to shield or guard the electron beam. Alternatively, the alternating field should be far removed from the beam to minimise any adverse effects.
The signal pick-up electrodes are connected to the input of an electronics amplifier for further signal processing. Known art of electronics engineering is used to separate the useful signal from unwanted effects.
These and further objects of the invention will be apparent from the following description of the invention as illustrated in the accompanying three Figures.